1. Field of the Invention
The present invention relates to the housing and protection of integrated circuit chips. More specifically, the present invention relates to a leadframe structure that offers enhanced thermal dissipation for integrated circuit chips.
2. Description of the Related Art
Modern integrated circuits (ICs) require protection from self-heating effects to operate reliably. One of the methods by which ICs are cooled is heat conduction through the leadframe. The leadframe typically consists of a metallic paddle on which the IC is mounted and lead fingers for providing electrically conductive paths from the bonding pads of the IC to external elements, such as circuit boards.
An example of a prior leadframe is illustrated in FIG. 1. The IC 10 is typically attached to a leadframe paddle 12. Electrically conductive lead fingers 14 are electrically connected to the IC 10 with wire bonds 16 and serve to provide electrically conductive paths from the IC 10 to external elements (not shown). The leadframe is typically encapsulated in a moulding compound 18. The external widths of the lead fingers 14 are typically standardized according to the type of package. For example, 8-lead dual in-line plastic (DIP) and 8-lead small outline integrated circuit (SOIC) packages have outer lead finger portions that are between approximately 0.33 and 0.51 mm wide.
The heat conduction paths are along extensions 20 of the leadframe paddle 12 and through the moulding compound 18, which typically has poor thermal conductivity when compared to the lead fingers (which are typically made of copper). Although the lead fingers 14 are generally excellent heat conductors,they contribute relatively little to the thermal dissipation because the distance and moulding compound between them and the paddle 12 lowers the thermal conductivity.
In addition to the heat dissipation limitations, the size of the IC 10 that this type of leadframe can accomodate is limited by the size of the leadframe paddle 12.
As IC fabrication technology advances, more power generating elements are incorporated into single ICs, creating a need for mounting packages with increased thermal dissipation properties. One prior leadframe design, described in U.S. Pat. No. 5,146,310 by Bayan et al., improves thermal dissipation by connecting the paddle to four thermally conductive pins and also by increasing the surface area of the leadframe paddle. The paddle pins allow the paddle to be connected to external heat sinks, such as a ground plane on a circuit board. Although the Bayan leadframe design increases thermal dissipation over standard leadframes, the pin count is increased by up to four relative to a standard package. In order to incorporate the Bayan design into standard package designs, such as 8-lead DIP or 8-lead SOIC packages, the package dimensions must be increased by up to 50 percent to accommodate the additional paddle pins. As a result, these packages must be redesigned to accommodate the Bayan leadframe structure.
Another leadframe structure, described in Japanese Patent Application No. 2-310954 by Isao Sasahara, increases thermal dissipation by placing the inner ends of the lead fingers in close proximity to the leadframe paddle and branching the outer ends of each lead finger, so that a portion of each lead finger is attached to a thermally conductive sheet positioned on top of the IC package. Although the Sasahara structure improves thermal dissipation over standard leadframe structures, additional tooling is required to trim and form the lead fingers after the leadframe has been encapsulated in the moulding compound, and additional manufacturing steps are required to apply the thermally conductive sheet to the top of the IC package.
A third leadframe structure, described in U.S. Pat. No. 4,943,843 by Okinaga et al., extends some of the lead fingers underneath the IC so that they are better anchored to the moulding compound that typically encapsulates the leadframe and IC. A side benefit of this design is increased thermal dissipation via the lead fingers that extend underneath the IC. However, an even higher level of heat transfer is desirable.